We revisit the water penetration and repellency of feathers and these factors' relation to bird behavior and habitat in the light of information that has become available since 1985. We present the physical principles of water penetration and repellency of porous surfaces and their relevance to feathers. We show that the requirements for water repellency of feathers in air and those for resistance to water penetration under water are partly in conflict with each other. Both aspects can be presented and evaluated by the same parameter, expressed in terms of the width 2r and spacing 2d of the barbs, (r d)/r. This parameter is small for water birds, particularly for those that dive frequently and are concerned with water penetration. It is larger for terrestrial birds, which are concerned primarily with water repellency. In each family of water birds, with some exceptions, a balance between water repellency and resistance to penetration exists as an adaptation to their particular habitat and behavioral pattern. For example, we discuss the habit of cormorants and darters of spreading their wings after a period in the water in relation to the value of the parameter. We postulate that information on habitat and behavior and, indirectly, the family identity of water birds, including those of fossil taxa, can be inferred from this parameter.
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